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Unravelling the Nature of the Active Species as well as the Doping Effect over Cu/Ce Based Catalyst for Carbon Monoxide Preferential Oxidation Jichang Lu, Jing Wang, Zou Qin, Dedong He, Liming Zhang, Zhizhi Xu, Sufang He, and Yongming Luo ACS Catal., Just Accepted Manuscript • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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ACS Catalysis
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Submitted to ACS Catalysis for publication
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Title:
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Unravelling the Nature of the Active Species as well as the Doping Effect over
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Cu/Ce Based Catalyst for Carbon Monoxide Preferential Oxidation
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Jichang Lu,† Jing Wang,†,‡ Zou Qin,† Dedong He,† Liming Zhang,† Zhizhi Xu,† Sufang
7
He,§ and Yongming Luo†,*
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†
Faculty of Environmental Science and Engineering, Kunming University of Science
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and Technology, Kunming 650500, P. R. China
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‡
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Hengyang 421001, P. R. China
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§
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Technology, Kunming 650093, P. R. China
College of Life Science and Environment, Research Hengyang Normal University,
Research Center for Analysis and Measurement, Kunming University of Science and
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ABSTRACT: The active sites of a mixed Cu/Ce material and the doping effect of
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typical element (iron, Fe) on the active species and the catalytic behavior of Cu/Ce for
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CO preferential oxidation in rich H2 (CO-PROX) were investigated by in-situ diffuse
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reflectance infrared fourier transform spectroscopy (DRIFTS), in-situ oxygen storage
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capacity measurement (OSC) combing with designed temperature-programmed
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reduction (TPR), along with Raman, X-ray photoelectron spectroscopy (XPS), X-ray
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diffraction (XRD) and temperature-programmed desorption/reduction of CO
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(CO-TPD/TPR). These results showed that two kinds of surface active center were
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involved in the CuCe and Fe doped CuCe systems, i.e., Cu+ as adsorption sites for the
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chemisorption and the activation of CO molecules, the surface reactive oxygen (the
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highly dispersed oxygen and surface lattice oxygen) that directly participated in the
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whole CO oxidation process. The addition of Fe into CuCe sample resulted in the
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incorporation of Fe into CeO2 lattice forming Fe-O-Ce structure and generated more
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oxygen vacancies, which not only enhanced the interaction between Cu and Ce to
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form more Cu+ absorption sites, but also trapped the gas-phase oxygen and promoted
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the release of subsurface lattice oxygen to supply more reactive oxygen. Thus, the
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turnover frequency (TOF) value was increased from 3.62×10-2 s-1 for CuCe to
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4.50×10-2 s-1 for Fe-doped CuCe. Moreover, with the enhancement of the lattice
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oxygen migration combining with the promotional role of Fe on the water gas shift
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(WGS), the capacity of the resistance to CO2 and H2O was enhanced for Fe-doping
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CuCe, and the corresponding stability time was largely prolonged from 170 h to 400 h,
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in the coexistence of CO2 and H2O. 2 ACS Paragon Plus Environment
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Keywords: CO-PROX, CuO-CeO2, active species, promotional role of Fe, stability.
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1. INTRODUCTION
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In the new energy scenario, with the increasing interest on the development of more
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clean energy with environmentally friendly and high-efficiency ways, proton
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exchange membrane fuel cells (PEMFCs) are considered as a promising source for
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energy in the future due to the advantages of the efficient, combustion-less and
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virtually pollution free.1 Hydrogen suitable to feed PEMFCs is normally generated by
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alcohols or hydrocarbons steam reforming followed by water gas shift reaction
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(WGSR).2 However, the gas stream obtained still contains a substantial level of CO
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(about 1%) which must be further reduced to 10 ppm in order to avoid poisoning of
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the platinum anode catalysts.3-5 The preferential oxidation of CO (CO-PROX) is
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deemed as a promising and potential solution due to its economic, practical and
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efficient characteristics for purifying H2-rich gases.3,6,7
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In general, two kinds of CO-PROX catalysts are mainly involved, i.e., supported
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noble metal materials and CuO-CeO2 based catalysts. Noble metal materials are one
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kind of catalysts with high-performance for CO-PROX, but they suffer from such
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obstacles as the high in price and the limitation in resource as well as low selectively
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at the high temperature.8-10 CuO-CeO2 is the other kind of the most researched
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catalytic system for the elimination of CO, which alternatives to noble metal catalytic
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systems for the best compromise between cost, activity, selectivity and thermally
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stability.6,11-14 The high catalytic activity of Cu/Ce catalysts is mainly assigned to the
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synergistic effect of copper and cerium oxides.15-18 It is known that CeO2 as the
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typical and promising support for various oxidation reactions is due to the following 4 ACS Paragon Plus Environment
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outstanding characteristics, one is the prominent redox ability of Ce4+/Ce3+ that can
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create the oxygen vacancies and induce the changes in the electron properties of
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active Cu species, and the other is the presence of abundant surface oxygen species,
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which usually involved in the oxidation reactions. Meanwhile, Cu species presented
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as Cu2+, Cu+, Cu0 over CeO2 surface possesses the well redox ability, and the
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synergistic interaction between Cu and Ce oxides enables Cuδ+ species to become the
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active adsorption sites for CO-PROX. However, since there are various kind of
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oxidized Cu species presented in the CuCe based sample owing to the
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above-mentioned features, for instance, CuOx clusters, bulk CuO, Cu2+ into CeO2
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lattice, Cu3+ and Cu-[Ox]-Ce structure, the surface redox properties of CuCe sample is
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usually complex. The origin of the catalytic activity and the relevant active center
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over CuCe catalysts for CO-PROX remains an open question in the reported literature.
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Some authors generally considered that the highly dispersed CuOx species is the
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catalytically active center for both CO-PROX and CO oxidation via the specific
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pretreatment (acid/base) for removing highly dispersed CuOx or eliminating the
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Cu-[Ox]-Ce structure.19-22 Strongly bound Cu-[Ox]-Ce layers was proved to promote
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the catalytic activity of CuCe for CO-PROX via providing the supply role.23-25
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Further, the specific Cu ions species was proposed and proved to be the active sites
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for CO-PROX. The researchers found by the DRIFTS spectra with the aid of parallel
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time-resolved X-ray diffraction (XRD) measurements under operando conditions that
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interfacial Cu+ species stabilized via the interaction between Cu and Ce oxides
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exhibited a strong correlation with the CO catalytic performance.26,27 Through the 5 ACS Paragon Plus Environment
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characterization of Fourier transformed extended X-ray absorption fine structure
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(EXAFS) spectra in the anaerobic process, the reduction of Cu2+ species into Cu1+ via
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the elimination of the first oxygen of Cu2+ was also demonstrated to make an
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important contribution to the CO-PROX reaction.28,29 Furthermore, recent research on
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the CO oxidation confirmed through X-ray absorption spectroscopy together with
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Density functional calculations that surface-substituted CuxCe1-xO2-y phase with the
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stabilization of Cu3+ and Cu2+ into host CeO2 lattice was the active phase and Cu3+
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was implicated in the mechanism for CO oxidation over Cu/Ce catalysts.30,31 It is not
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surprising because the heterogeneous catalysis is totally a complex processes, even for
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the relatively simple reactions system, which results in the fact that the catalytic
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activity is not only limited by the difference in the adsorption properties of CO over
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different Cuδ+ species, but also depends on the surface redox chemistry, the active
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oxygen properties, and oxygen vacancies and so on. In view of those, the
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understanding of the roles these Cu ions play in the reaction of CO-PROX (specific
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catalytic active center) over CuO-CeO2 catalysts is still in progress. And more notably,
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little attention has been extensively paid to the significance of the active oxygen
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species on the reaction process of CO-PROX.26,32
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More importantly, any adjustment or modification in nanoscale via the doping of
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different types of additives or the preparation with different methods could evoke the
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unexpected variation in the catalytic behavior.33 As an typical promoter, Fe oxides
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has the high oxygen storage capacity as similar to CeO2. It should be noted that Fe
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itself also possesses high redox ability as well as the lower redox potential of 6 ACS Paragon Plus Environment
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Fe2+/Fe3+ (0.77 V) than that of Ce3+/Ce4+ (1.61 V),34 thus the interaction between
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Fe2+/Fe3+, Cu2+/Cu+, and Ce3+/Ce4+ makes the changes in the structural and electronic
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properties to be more complicated. Thus, Fe species is selected as the model additives
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to investigate the doping effect on the active center over CuCe sample for CO-PROX.
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Although large number of researches focused on the research of the catalytic
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properties over Cu/Ce catalysts, the in-depth insight into the catalytic behavior for the
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modification of Cu/Ce catalysts by iron (Fe) species in the reaction process of
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CO-PROX (especially the tolerance of H2O and CO2 as well as the stability) is
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relatively less. At present, for the selective CO oxidation, some authors found that the
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incorporation of Fe species into Cu/Ce catalysts prepared by wet impregnation and
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single-step citrate method generated better reducibility to improve the interaction
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between Cu and Ce, which leads to the higher dispersion of copper, thus posing the
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positive effect on the CO oxidation activity.35,36 Other researchers deemed that the
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introduction of Fe additives into Cu/Ce catalysts in the certain preparation procedure
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promoted the generation of additional amount of active sites related to CuO due to the
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electron-ion interactions, which improved the CO-PROX activity.37 However, the
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negative CO oxidation activity was reported for using CuO-CeO2 catalysts doped with
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Fe species when the co-precipitated method was applied.38 They deemed that the
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reversible desorption of absorbed CO was not beneficial to the selective CO oxidation
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via the characterization of in situ DRIFTS and XPS techniques. Moreover, in the field
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of soot combustion,39,40
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chemical-looping reforming,41 the water gas shift (WGS) reaction,42 Fe species was
selective catalytic reduction of NH3 with NO,34
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extensively applied to modify the redox property of pure CeO2 catalysts, and they
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found that the electronic inductive effect between Fe3+ and Ce4+ might enhance the
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redox properties for various catalytic activity of above reactions, sometimes involving
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the generation of Fe-O-Ce structure when prepared by surfactant-assisted and
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coprecipitation method.34,40 In a word, the modification of Fe additive into CuCe
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sample prepared by the different methods could largely give rise to the change in the
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catalytic behaviors. Therefore, the investigation of the doping influence of Fe additive
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on the structure, the catalytic active sites and the catalytic behavior (the tolerance of
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H2O and CO2 as well as the stability) of Cu/Ce catalyst system as well as the catalytic
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active species over Fe doping CuCe catalysts for CO-PROX are of great necessity.
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In our previous literature, the different ratio of Fe/Cu and the different Fe
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precursors were selected to be doped on the CuO-Ce0.8Zr0.2O2 materials for
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CO-PROX.43 In this work, simplified CuO-CeO2 and Fe doped CuO-CeO2 materials
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were prepared by the simple and solvent-free route of combustion synthesis. With the
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aid of designed in-situ TPR and the CO pulsing technique as well as in-situ DRIFTS,
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the catalytic active center for CO-PROX over CuCe and CuCe-Fe were illuminated
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from
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composition-structure-activity-stability relationships for the introduction of Fe into
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CuCe sample were well understood. The long-term stability of about 400 h was
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obtained for Fe doped CuCe sample in the presence of H2O and CO2, which was twice
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as stability as CuCe sample. This results can provide insights into understanding the
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active sites over doped CuO-CeO2 based materials for CO-PROX and give guidelines
the
point
of
the
principle
of
heterogeneous
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the
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for the practical applications.
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2. MATERIALS AND METHODS
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2.1. Catalyst Preparation. CuO-CeO2 catalyst was synthesized with a solvent-free
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combustion synthesis (SFCS) method. The method was briefly depicted in the
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following. Firstly, the calculated amount of Copper nitrate hydrate (Cu(NO3)2·3H2O,
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AR), Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, AR) and Carbamide
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(CO(NH2)2, AR) were added into an agate mortar without additional water, and then
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ground completely for 10 min, where a transparent and homogeneous viscous gel was
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obtained. Subsequently, the as-obtained gel was calcined at 700 ºC for 20 min,
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denoted as CuCe, where the corresponding content of CuO was 10 wt. %. Fe-doped
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Cu/Ce sample was also prepared by mixing and grinding the Iron nitrate nonahydrate
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(Fe(NO3)3·9H2O, AR) with above-mentioned chemicals. The Fe/Cu mass ratio was
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0.025, and the subsequent procedures were the same as described by CuCe catalyst,
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denoted as CuCe-Fe.
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2.2. Conventional characterizations. XRD patterns were performed on a Rigaku
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D/max-1200 instrument equipped with Cu Kα radiation at 2θ ranges between 10 and
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90o. Peak identification was conducted by the software of Jade 6. Transmission
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Electron Microscopy (TEM) was performed by using a Tecnai G2 TF30 S-Twin
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microscope operated with an accelerating voltage of 300 kV. Nitrogen sorption
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isotherms were determined on a V-Sorb 2800P instrument at 77 K. Before
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measurement, the samples were degassed at 200 ºC under vacuum for 3 h. X-ray
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photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe II with 9 ACS Paragon Plus Environment
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non-monochromatic Al Kα radiation (1486.6 eV) as the excitation X-ray source. The
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chemical states and the corresponding surface element compositions of all the samples
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were determined by the Casa XPS software. The charge calibration was applied by
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setting the main C 1s peak at 284.6 eV. Raman spectra were detected by using a Via
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Reflex Confocal Raman Microscope with a laser (514 nm) working at 5 mV power.
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The common experiments of hydrogen temperature programmed reduction (H2-TPR)
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were generally carried out in a catalyst bed with a quartz tube reactor equiped with a
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thermal conductivity detector (TCD). Before each measurement, the prepared samples
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were pretreated in pure O2 at 400 °C for 30 min. 100 mg of the pretreated samples
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were heated in a gas flow (30 cm3/min) of 10% H2 in Ar from 50 to 400 ºC (800 ºC) at
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a ramp rate of 10 ºC/min.
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Oxygen storage capacity (OSC) of samples was determined by the CO pulsing
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technique. The micro-reactor equipped with the six-way gas-sampling valve with a
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measuring ring (50 μmol/L) was used to introduce the quantitative gas sample. Before
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testing, the sample (50 mg, 100-120 mesh) was pretreated at 550 oC for 60 min in the
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flow of 20% (v/v)O2/He with the velocity of 30 mL/min. Then, the samples were
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cooled down to the desired temperature (150 oC and 200 oC, respectively) under the
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flow gas of high-purity helium (He). Afterwards, the pure CO gas sample (99.99%)
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was introduced every 5 min into the micro-reactor until reaching the plateau. The
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detection of CO and CO2 was performed by the on-line detector equipped with a
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high-sensitivity of TCD and a flame ionization detector (FID) together with a
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methanation reactor. To verify the presence of the deposited carbon over samples 10 ACS Paragon Plus Environment
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surface, the CO pulse was introduced at 140 oC over CuCe and CuCe-Fe samples (100
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mg) until the amount of CO reaches the steady state. The samples were denoted as
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pulsed CuCe and pulsed CuCe-Fe, respectively.
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Temperature programmed desorption of CO (CO-TPD) was performed on the same
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apparatus with TCD detector. 100 mg of samples (100-120 mesh) were firstly
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pretreated under the 20% O2/He at 400 oC for 1 h. After that, the temperature was
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cooled down to 30 oC, and the flow gas was switched to 100% CO (30 ml/min).
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Afterwards, the pretreated samples were saturated using pure CO at 30 oC for 30 min,
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then were purged with He (30 mL/min) for 60 min to remove the physisorbed CO.
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The CO-TPD experiment was carried out by elevating temperature up from 30 to 300
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o
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gas of He into pure N2 to detect the product of H2 with TCD detector. 200 mg of
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samples were exposed to 10% CO/N2 at 30 oC for 30 min with a flow rate of 30
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mL/min. Then, the temperature was ramped from 30 to 300 oC at a ramp rate of 1
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o
210
valve.
C with a heating rate of 10 oC/min. CO-TPR was carried out by switching the carrier
C/min, and the gas was analyzed every 5 min assisted with a six-way gas-sampling
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Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was
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conducted on a Nicolet 6700 spectrophotometer equipped with an mercury cadmium
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telluride (MCT) detector with a resolution of 4 cm-1. A diffuse reflection infrared cell
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was connected to a vacuum apparatus, and was fitted with temperature controlled
215
parts and a CaF2 window. Prior to the measurements, the samples were firstly
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pretreated at 300 oC for 30 min under the atmosphere of 10% O2/Ar. Background 11 ACS Paragon Plus Environment
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spectra were subtracted at the target temperature under the high-purity Ar atmosphere
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by averaging 32 scans. For every measurement, DRIFTS spectra were obtained for
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samples under the feed gas of 1% CO/1% O2/50% H2/He and then recorded from 30
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to 210 oC by every 20 oC. To investigate the deactivation reason caused by the
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addition of H2O, small amount of H2O was introduced to system via carried by the
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reaction gas stream through bubbling, in-situ DRIFTS spectra were performed over
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CuCe and FeCuCe catalysts from 90 oC to 210 oC by every 20 oC.
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2.3. Designed characterizations. Four kinds of designed H2-TPR over CuCe and
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CuCe-Fe samples were carried out and classified as follows: 1) the elimination of
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surface oxygen species (β peak-TPR). To effectively eliminate α peak corresponding
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to the highly dispersed CuOx species and avoid the thermal effect, the ramp rate was
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decreased from 10 to 5 ºC/min, the samples (100 mg) were firstly pretreated with 10%
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H2/Ar (v/v, 30mL/min) at 150 oC for 0, 1, 2, 3, 5 min, respectively, denoted as P150-0,
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P150-1, P150-2, P150-3, P150-5. Then, the gas mixtures was switched into the
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atmosphere of high-purity He followed by decreasing the pretreatment temperature
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from 150 oC to the room temperature. These H2-TPR measurement were finally
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conducted with the procedure as same as above-mentioned TPR process. The optimal
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pretreatment conditions were found at 150 oC by 2 min for CuCe (P150-2) and by 1
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min for CuCe-Fe sample (P150-1); 2) the in-situ characterization of the active oxygen
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species of above pretreated sample (pretreatment-OSC). CuCe and CuCe-Fe samples
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were firstly pretreated using 10% H2/Ar at 150 oC for 2 min and for 1 min,
238
respectively. Then, the corresponding temperature was decreasing to 30 oC and the 12 ACS Paragon Plus Environment
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flow gas was switched to the flow of He to completely remove the H2/Ar gas.
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Subsequently, the OSC measurement of pretreated samples was performed at the
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temperature of 200 oC. 3) the ex-situ characterization of the oxygen migration
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(Activity-TPR). Firstly, the samples (100 mg) were pretreated by 10% H2/Ar to
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eliminate α peak as did in the Section 1). Then, the normal catalytic activity tests of
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pretreated samples were performed under the feed gas of 1% CO/1% O2/50% H2/He
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(80 mL/min) from 60 to 160 oC (P150-R160) and 60 to 220 oC (P150-R220),
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respectively. Afterwards, after the temperature was declined under the high-purity He,
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the spent CuCe and CuCe-Fe samples were rapidly transferred to conduct the H2-TPR
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measurement,
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P150-1-R220), respectively. 4) the in-situ characterization of the oxygen migration
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(OSC-TPR). Firstly, the samples were pretreated under the flow of 20% O2/He (30
251
mL/min) at 550 oC for 60 min. After switching into the high-purity He and decreasing
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the temperature to 200 oC. The OSC measurement of samples was carried out at 200
253
o
254
consume the surface initial active oxygen. Then, the temperature was declined to 50
255
o
256
to the 10% H2/Ar, and the in-situ TPR experiments were conducted from 50 to 400 oC
257
with a ramping rate of 10 oC/min.
denoted
as
(P150-2-R160,
P150-2-R220) and
(P150-1-R160,
C where only one CO pulse with a measuring ring of 50 μmol/L was introduced to
C under the atmosphere of high-purity He. Subsequently, the flow gas was switched
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2.4. Catalytic activity tests. The selective oxidation of CO in H2-rich gas was
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conducted on a tubular reactor at the temperature between 60-220 ºC with the
260
atmospheric pressure. 300 mg of catalyst (sieve fraction, 40-60 mesh) was placed in a 13 ACS Paragon Plus Environment
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quartz tubular reactor. The synthetic gas (containing 1% CO, 1% O2, 50% H2 in
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volume and He as balance gas) was introduced to fix a total flow velocity of 80
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mL/min. To investigate the influence of CO2 and H2O on the catalytic activity and
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stability, the experiments for the resistance to CO2 and H2O over two samples were
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performed by introducing 15% CO2 and 10% H2O into the feed gas to arrive the total
266
flow rate of 80 mL/min. The stability tests were conducted at 160 oC for CuCe and
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155 oC for FeCuCe with the same other conditions. Moreover, the independent WGS
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reaction tests were performed over CuCe and FeCuCe samples at the temperature
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range of 120-260 oC using the mixtures of 1% CO, 10% H2O, and pure N2 as balance
270
(80 mL/min). The WGS reactions over pulsed CuCe and CuCe-Fe samples (100 mg,
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the carbon species was formed) were also conducted using the mixtures of 10% H2O
272
and pure N2 as balance with the same flow rate.
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In order to research the essential role of the different surface active oxygen species
274
and the oxygen mobility, two kinds of specific activity tests were also designed. For
275
the first activity test, 100 mg of fresh CuCe and CuCe-Fe samples were performed
276
under the same feed gas composition with the unchanged flow rate. With respect to
277
the second activity test, 100 mg of fresh CuCe and CuCe-Fe samples were firstly
278
pretreated under the flow gas of 10% H2/Ar at 150 oC for 2 min and 1 min,
279
respectively, in order to remove the α peak corresponding to the highly dispersed
280
oxygen in CuOx species. Then, the flow gas was switched into the atmosphere of
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high-purity He, and the temperature was declined to 60 oC. Afterwards, the mixture
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gas (1% CO/1% O2/50% H2/He) was introduced to conduct the other normal activity 14 ACS Paragon Plus Environment
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test.
284
Every activity tests were conducted when the steady state was arrived at the
285
corresponding temperature, and were ran at least six times on each temperature and
286
then took the average. The inlet and outlet streams of various reactions were
287
determined by using an on-line gas chromatography (GC), which was equipped with a
288
TCD detector with different carrier gas depending on the detected products. CO
289
conversion (%) and CO2 selectivity (%) were calculated according to CO and O2
290
consumption, as described below:
291
CO conversion% [CO]in - [CO]out 100 [CO]in
292
CO 2 selectivit y % 0.5 [CO] in - [CO] out 100 [O 2 ]in - [O 2 ]out
(1) (2)
293
To calculate the the turnover frequency (TOF) of samples, the internal/external
294
diffusion effects could be excluded by making a Weisz-Prater analysis and a Mears
295
analysis. On the basis of the assumption of hemispheric model over CuCe based
296
catalysts, TOF values of catalysts for CO-PROX were calculated according to the
297
following equations:44,45
298
TOF (s -1 ) FCO xCO mcat /( DCuO nCuO )
299
where mcat is the weight of catalyst (g), FCO is the flow velocity of feed gas (mol s-1),
300
χCO is the CO conversion, which was kept below 10% at 80 oC by increasing the flow
301
rate assisted by decreasing the amount of catalysts, nCuO is the total molar amount of
302
CuO (mol), DCuO is the dispersion of CuO, which was calculated based on the ratio of
303
Cu/Ce from XPS results.
304
3. RESULTS AND DISCUSSION
1
(3)
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305
3.1. Catalytic activities of CuCe based materials. Fig. 1 displays the conversion
306
of CO and the selectivity of CO2 during the process of CO-PROX over the CuCe and
307
CuCe-Fe samples. From Fig. 1A, CuCe-Fe catalyst exhibits the higher catalytic
308
performance than that of CuCe. The reaction temperatures T50 and T100
309
(corresponding to 50% and 100% of CO conversion) are introduced as the important
310
criterion to evaluate the CO oxidation activity. It can be seen that the reaction
311
temperature for T50 is 69, 77 ºC, and the corresponding temperature for T100 is 99, 120
312
ºC over CuCe-Fe, CuCe, respectively. The corresponding temperature window widens
313
from 40 ºC (120-160 ºC) for CuCe to 60 ºC (100-160 ºC) for CuCe-Fe. Those results
314
indicates that the doping of Fe into CuCe catalyst promotes the CO oxidation activity,
315
especially for the low temperature activity. The turnover frequency (TOF) and the
316
reaction rate are calculated to estimate the CO oxidation activity, and the results of
317
CuCe-Fe and CuCe samples are complied in Table 1. It is found that the incorporation
318
of Fe increases the corresponding TOF value (and the reaction rate) from 3.62×10-2 s-1
319
(1.13×10-6 mol·g-1·s-1) to 4.50×10-2 s-1 (1.60×10-6 mol·g-1·s-1), testifying the
320
promotional role of Fe species on the catalytic activity of CuCe catalyst for
321
CO-PROX. With further rising the temperature from 160 to 220 oC, the conversion of
322
CO for all the catalysts declines to the different degree due to the rapid oxygen
323
consumption rate at the high temperature as well as the occurrence of hydrogen
324
oxidation. The selectivities of CO2 for two catalysts are shown in Fig. 1B. As seen,
325
with increasing the reaction temperature, the selectivity of CO2 for the catalysts
326
gradually decreases, probably owing to the possible competing reaction for converting 16 ACS Paragon Plus Environment
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327
H2 and O2 to H2O. It can be concluded that the addition of Fe have evidential
328
promotional effect on the catalytic activity of CuCe for CO-PROX. Therefore, the
329
following characterizations are used to investigate the promotional effect of Fe
330
additive on the structural, textural, and electronic properties of CuCe catalyst for
331
CO-PROX.
332
3.2. Structural, textural and electronic properties of CuCe and CuCeFe
333
materials. The structural property of catalyst is studied by X-ray diffraction (XRD).
334
The corresponding patterns of CuCe and CuCe-Fe samples are shown in Fig. 2A. The
335
main crystallite size and the lattice parameter of CeO2 particles over two catalysts are
336
calculated and listed in Table 1. The small and weak diffraction peaks at 35.2 and
337
38.5o attributed to copper oxides (CuOx) are visible for CuCe sample, indicating that
338
large number of CuOx species is aggregated on the surface to be detected by XRD and
339
a small portion of CuOx is incorporated into CeO2 lattice (the lower unit cell
340
parameter for CuCe (0.54126 nm) than that for pure CeO2 (0.54131 nm)). However,
341
the characteristic peaks of CuOx are not discovered in the XRD pattern of CuCe-Fe
342
catalyst. Two reasons could be probably considered to interpret this phenomenon, one
343
can expect that the doping of Fe species might result in the incorporation of more
344
CuOx species into CeO2 lattice forming the solid solution. The other can expect that
345
the incorporation of Fe species into CeO2 lattice by replacing Cu species is presented,
346
which facilitates the highly redispersion of more CuOx species over the CeO2 surface,
347
forming the stronger interacting of Cu with Ce.46,47 Compared with the diffraction
348
diagram of CuCe, the (111) peak shifts to a higher location for the CuCe-Fe (Fig.2, 17 ACS Paragon Plus Environment
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349
inset). Meanwhile, the unit cell parameters calculated were 0.54126 for CuCe and
350
0.54016 nm for CuCe-Fe sample, respectively. This lattice shrinkage and the shift of
351
the position of the (111) peak demonstrate the consequence of the incorporation of
352
iron or more copper species into the CeO2 lattice, due to the smaller Fe (II 0.075 nm;
353
III 0.064 nm) and Cu(II 0.073 nm) ionic radius compared to that of Ce(IV 0.097
354
nm).22 This incorporation effect might result in the formation of the distortion in the
355
cubic fluorite structure of CeO2, thus generating more defective sites.48 The ratio of
356
Cu/Ce evaluated by the measurement of XPS (Fig.2) is used to characterize the
357
distribution state of Cu species over CeO2. The corresponding result is summarized in
358
Table 1. The Cu/Ce atomic ratios are calculated to be 0.293 and 0.336 for CuCe and
359
CuCe-Fe, respectively. The higher value for CuCe-Fe indicates the generation of more
360
amount of surface dispersed Cu oxide species, demonstrating that the addition of Fe
361
species gives rise to the incorporation of Fe into CeO2 lattice, which leads to the
362
migration and redistribution of partial Cu oxide species from CeO2 bulk lattice to
363
surface and interface (as evidenced by the higher intensity of α and β TPR peaks).
364
Moreover, the main reflections (2θ=28.5, 33.1, 47.5, 56.3, 59.1, 69.4, 76.7 and
365
79.1o) corresponding to the fluorite-type cubic structure of CeO2 with diffractions of
366
(111), (200), (220), (311), (222), (400), (331) and (420) planes (ICDD 34-0394) are
367
observed for both CuCe and CuCe-Fe samples.49-51 It is seen Fig. 2A (insert) that the
368
characteristic peaks of CeO2 over CuCe-Fe are much broader and have a lower
369
intensity than those of CuCe, suggesting the formation of smaller-sized CeO2
370
crystallites. As seen from Table 1, the corresponding crystallite size is calculated to be 18 ACS Paragon Plus Environment
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371
9.6 nm for CuCe-Fe, smaller than that of CuCe (11.0 nm), suggesting the addition of
372
Fe has a certain influence on the crystallite size of CeO2, due to the fact that the
373
distortion of CeO2 fluorite-type cubic phase caused by Fe additive inhibits the
374
excessive growth of particles clusters. The textural properties of the CuCe and
375
CuCe-Fe catalysts were listed in Table 1. The specific surface area (SBET) of CuCe-Fe
376
sample is higher than that of CuCe, suggesting that the modification of Fe causes an
377
increase in the surface area of the resultant catalyst (CuCe-Fe), which is due to the
378
smaller crystallite size of CuCe-Fe. TEM characterization is used to analyse the
379
morphological feature of CuCe and CuCe-Fe, as shown in Fig. S1-A and Fig. S1-B,
380
and the particles size distribution is shown in the corresponding inserts. TEM images
381
display the existence of near-spherical, agglomerated particles in two samples. The
382
average sizes are calculated to be 9.7 nm for CuCe-Fe and 10.7 nm for CuCe sample,
383
respectively, consistent with the XRD result. Furthermore, CuCe-Fe sample possesses
384
the relative narrow and uniform particles size distribution compared to that CuCe
385
sample, demonstrating that the addition of Fe improves the dispersion of CuCe
386
sample.
387
The XPS spectrum is employed to acquire further information about the surface
388
composition and the oxidation/valence state of Ce, Fe and CuOx species. Fig. 2B, 2C
389
and 2D display the Ce 3d, Fe 2p and Cu 2p signal of CuCe and CuCe-Fe samples,
390
respectively. As shown in Fig. 2B, the Ce 3d XPS spectra of CuCe and CuCe-Fe
391
samples are complex and contain four spin-orbit doublets. Three doublets marked as v
392
(ca. 882.2 eV) - u (ca. 900.7 eV), vˊˊ (ca. 888.6 eV) - uˊˊ (ca. 907.6 eV) and vˊˊˊ (ca. 19 ACS Paragon Plus Environment
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393
898.3 eV) - uˊˊˊ (ca. 916.5 eV) are attributed to the presence of Ce4+ species. Letters v
394
and u refer to the 3d5/2 and 3d3/2 spin-orbit components, respectively. Meanwhile, for a
395
practical simplification, two components of vˊ (ca. 884.5 eV) and uˊ (ca. 903.0 eV)
396
are ascribed to the existence of Ce3+ species.52 Those results indicates Ce3+ and Ce4+
397
species are simultaneously presented in the CuCe and CuCe-Fe samples. For Fe
398
species, as shown in the Fig. 2C, the broaden and asymmetric peaks suggests that both
399
Fe2+ and Fe3+ co-exist in the CuCe-Fe sample. The binding energy at 710.0 eV for Fe
400
2p3/2 sates followed with a satellite around 716.3 eV is attributed to the presence of
401
Fe2+ species, another peak around 713.4 eV with a corresponding statellite at 719.4 eV
402
is assigned to the Fe3+ species.53,54 This Fe3+ species is found to be shifted to high
403
value compared to that reported for Fe3+ around 712-713 eV in the literature,53,54
404
suggesting the change in the electron environment around Fe species. This is possibly
405
due to the fact that the incorporation of Fe species into CeO2 lattice enhances the
406
interaction between Fe and Ce leading to the formation of Fe-O-Ce structure.34,39
407
Furthermore, the intensity ratios of the satellite to main peak of Fe 2p3/2 are calculated
408
to be 1.70 for Fe2+ and 4.18 for Fe3+ species, respectively, which is higher than that
409
reported in literature (blew 1).55 As inferred from the literature,55,56 according to
410
Harrison’s relation, the intensity of the satellite peak of iron oxides is proportional to
411
the value of 1-1/R7.0, where R is the interatomic distance of Fe-O bond (Section 2 of
412
supporting information (SI)). Since the incorporation of Fe species into CeO2 lattice
413
leads to the increment in the length of Fe-O bond due to the difference of ionic radius
414
between Fe2+ (0.075 nm), Fe3+ (0.064 nm) and Ce3+ (0.097 nm), the satellite intensities 20 ACS Paragon Plus Environment
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415
thus significantly increase, which, in turn, proves the formation of Fe-O-Ce structure.
416
From Fig. 2D, a main and asymmetric Cu 2p3/2 peak at around 930-936 eV is found
417
for CuCe sample, which can be decomposed in two contributions centered at 932.2 eV
418
and 933.4 eV, generally associated with the Cu+ and Cu2+ species, respectively.57-59
419
This suggests that the coexistence of both Cu2+ and reduced Cu+ species in the CuCe
420
sample where CuOx species is mainly presented in the form of reduced Cu+ species.
421
The almost inexistence of satellite peaks that is located at 938-946 eV for Cu2+ seems
422
to confirm the existence of main Cu+ species. The intensity ratio of the shake-up peak
423
to the principal peak (Isat/Ipp) is used to characterize the valence state of Cu species. It
424
is generally accepted that the value below 0.57 is corresponding to Cu+ species and
425
the higher value is assigned to Cu2+ species.22,28 The corresponding values are
426
compiled in Table 1. CuCe and CuCe-Fe samples display the ratios of 0.19 and 0.32,
427
proving the existence of Cu+ species. Cu LMM Auger spectra was performed to
428
confirm the presence of reduced Cu oxide species, as shown in the inset of Fig. 2(D).
429
It is seen that the binding energy at 569.7 eV is attributed typically to the existence of
430
Cu+ species, followed by two other peaks representing different Auger transitions
431
involving Cu+, and no distinct signal assigned to Cu0 species at about 568 eV is
432
observed, demonstrating the presence of the main valence state of reduced Cu species
433
as Cu+.60,61 The reduced copper species on the sample may be originated from the
434
strong interaction between CeO2 and the active Cu components,62 because of the
435
existence of the redox balance (Ce3+ + Cu2+ ↔ Ce4+ + Cu1+),32 as confirmed by redox
436
Ce4+/Ce3+ in the Ce 3p XPS. After the addition of Fe species, the redox properties of 21 ACS Paragon Plus Environment
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437
CuCe species has changed largely. It can be seen that the Cu 2p3/2 peak are broadened
438
and more Cu2+ ion are formed, possibly due to the additional redox equilibrium (Fe3+
439
+ Cu+ ↔ Fe2+ + Cu2+). The structural and electronic impact of Fe additive on the
440
CuCe sample are further demonstrated by Raman and O 1s XPS analysis.
441
Raman spectroscopy is a useful tool to characterize the vibration of metal-oxygen
442
bond, the defects of nanophase as well as their surrounding chemical environment. It
443
has proven that the doping phenomenon caused by electrons was associated with the
444
change in the position of Raman band.63,64 Therefore, the doping effect of Fe species
445
on the CuCe sample is characterized by Raman analysis, and the corresponding
446
spectra of CuCe and CuCe-Fe catalysts are shown in Fig. 3A. The CuCe sample
447
displays an intense band at about 463 cm-1, attributed to the Raman active F2g
448
vibrational mode for oxygen atoms around the cerium ions, which is an evidence for
449
the typical cubic fluorite structure CeO2.64,65 The broad peak around 599 cm-1 can be
450
linked to the oxygen vacancies/defects within the lattice of CeO2.66,67 When the Fe
451
species is added, the intense band is observed to be shifted slightly toward the low
452
frequency at 459 cm-1. This phenomenon is attributed to the fact that the generation of
453
Fe-O-Ce structure gives rise to the change in the metal-oxygen bond lengths owing to
454
the difference in the atomic radius between Fe3+ and Ce4+/Ce3+, forming the lattice
455
distortions.34,64 The increase of lattice distortions indicates the formation of more
456
oxygen vacancies. The oxygen vacancies concentration is calculated with the nature
457
of a major peak at 459 cm-1 by the spatial correlation model,64,68,69 as shown in
458
Section 3 of SI. The defect concentration of CuCe and CuCe-Fe is calculated to be 22 ACS Paragon Plus Environment
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459
3.84 × 1021 cm-3 and 4.43 × 1021 cm-3, respectively (listed in Table 1), suggesting that
460
the addition of Fe species increases the amount of oxygen vacancies, which also
461
proves the Fe doping into CeO2 lattice to generate Fe-O-Ce structure. Moreover, the
462
absence of any Raman band for Fe species at the low Raman band (217, 296, 398 cm-1)
463
also confirms this point.70 The further evidence for Fe doping into CeO2 lattice to
464
form Fe-O-Ce coordination could be achieved by looking at the binding energy of O
465
1s. Fig. 3B presents a comparison of O 1s signal of CuCe and CuCe-Fe samples. The
466
main binding energy peak centered at 528.9 eV and a shoulder peak around 531.3 eV
467
are observed for CuCe sample. The main peak is ascribed to the lattice oxygen (OI),
468
and the shoulder peak reflects chemisorbed oxygen (OII) in carbonates and hydroxyl
469
groups.71,72 Upon Fe doping, it can be seen that the main peak shifts to a higher value
470
at 529.3 eV, which could be associated with the transformation of O chemical
471
environment from original Ce-O-Ce to Fe-O-Ce coordination (Fe containing
472
CeO2-like solid solution).66,73 In a word, the structural model of CuCe as well as the
473
promotional effect of Fe additives on the structural property of CuCe are well
474
illuminated in Scheme 1.
475
3.3. Fe promotional effect on the oxygen mobility. The structure-activity
476
relationship over CuCe materials has been previously reported and discussed
477
extensively by large amount of researchers.19-31 Although the presence of various
478
viewpoints on the cognition of the active species over CuCe catalysts, all these
479
researches provide insight into the possible critical factors affecting the final catalytic
480
performances from different views via different methods and techniques. Thus, we 23 ACS Paragon Plus Environment
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481
would take the active sites of CuCe based catalysts into account from the view of the
482
principle of heterogeneous catalysis. In general, the catalytic reaction process
483
commonly involved, firstly, the adsorption and the activation of reactant molecules,
484
then the reaction between reactants over the active sites, as well as the desorption of
485
products. As for CO-PROX (including CO oxidation and H2 oxidation) and CO
486
oxidation, it was generally accepted that CO oxidation started with the chemisorption
487
and the activation of CO molecules over Cu ions active sites, then the reaction
488
between Cu ions-CO species and surface active oxygen species over Cu-Ce oxides
489
formed CO2 and leaved an oxygen vacancy, finally the formed oxygen vacancies were
490
replenished by gas-phase oxygen, which was known to be a Mars-van Krevelen
491
mechanism.27,74-77 Therefore, the catalytic activity of CuCe based materials toward
492
CO oxidation in H2-rich is speculated to be largely affected by two significant factors,
493
one is the surface active oxygen species, and the other is the Cu ions active sites.
494
Liable/reducible oxygen species is usually involved in the oxidation reactions over
495
oxide catalysts.74,75 In general, when an reactant is oxidized at the active surface, the
496
oxidant is usually a surface lattice oxygen atom instead of gas-phase oxygen, then
497
creating a surface oxygen vacancy.78,79 The production of oxygen vacancies plays an
498
important role in the formation of mobile oxygen species. As is known that the
499
amount of oxygen vacancies can be estimated by several parameters, such as the ratio
500
of Ce3+/(Ce3++Ce4+) from Ce 3d XPS,80 the value of A599/A600 for Raman,81 and the
501
ratio of OII/(OI+OII) from O 1s XPS.82 According to the preceding result, these values
502
are calculated to be 27.56, 0.67, 51.65 for CuCe-Fe sample, which is totally larger 24 ACS Paragon Plus Environment
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ACS Catalysis
503
than that of 22.0, 0.60, 36.33, respectively, for CuCe sample. This would demonstrate
504
that the addition of Fe into CuCe sample leads to the generation of more amount of
505
oxygen vacancies. The presence of oxygen vacancies possibly enhances the activity of
506
oxygen (which can be evaluated by the oxygen mobility ability). The oxygen mobility
507
promoted by Fe additive is further demonstrated by designed H2-TPR and OSC
508
characterizations.
509
Temperature-programmed reduction by hydrogen (H2-TPR) is actually used to
510
characterize the presence of reducible/active oxygen species due to the consumption
511
of oxygen by H2 to produce water in the TPR process. The common H2-TPR profiles
512
of the CuCe and CuCe-Fe catalysts are shown in Figure 4A. For CuCe sample,
513
H2-TPR profile show three peaks at 161 ºC (α), around 233 ºC (β), and 250 ºC (γ). It
514
is generally accepted that the small α peak could be assigned to the consumption of
515
reducible oxygen from the highly dispersed CuOx species, large β peak can be
516
ascribed to the reduction of active oxygen in the strong interaction of Cu-[Ox]-Ce (or
517
Fe-O-Ce) species, and small γ peak is attributed to the oxygen species presented in
518
bulk CuO.19,23,83 Compared to the oxygen consumption of pure CuO characterized by
519
one peak at 280-350 oC (Section 4 of SI) and between 300-400 oC in the literature,20,84
520
the decreased reduction temperature for CuCe sample indicates that the oxygen
521
species linked to Cu is activated by the interaction with CeO2. Upon the doping Fe
522
into CuCe sample, the α and β peak slightly shift to a lower temperature and the
523
corresponding H2 consumption increases from 792.6 μmol/g for CuCe to 937.9
524
μmol/g for FeCuCe, indicating that Fe additive activates the surface lattice oxygen in 25 ACS Paragon Plus Environment
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525
CeO2, promoting the oxygen migration. In other words, as before proved that Fe
526
doping into CeO2 lattice results in the creation of more oxygen vacancies, the
527
presented oxygen vacancies possibly promote the migration ability of oxygen from
528
CeO2 subsurface to surface Cu-[Ox]-Ce and from surface to the highly dispersed CuOx,
529
and thus the generation of more highly reducible oxygen species,85,86 as shown in
530
Scheme 1. The TPR peak in the higher temperature region in Fig.4A(a) supports the
531
conclusion. It is seen that after the incorporation of Fe additive, the H2 consumption
532
peak at around 400 oC corresponding to the subsurface lattice oxygen over CuO-CeO2
533
(surface lattice oxygen of CeO2) disappears, and the higher intensity of peak centered
534
around 780 oC characterized by the bulk lattice oxygen of CeO2 over Fe-CuCe (464.4
535
μmol/g) compared to CuCe sample (390.5 μmol/g) is observed, indicating that Fe
536
doping facilitates the migration of subsurface lattice oxygen as well as the release of
537
bulk lattice oxygen over CuO-CeO2.
538
It is deduced that the enhanced performance for the CO catalytic oxidation using
539
CuO-CeO2 as the catalyst depends on the ability to provide the reactive oxygen to the
540
reaction process. Thus, the roles the oxygen mobility plays in the reaction process
541
becomes even more significant. The oxygen release behavior and the oxygen mobility
542
are further characterized by the CO pulsing technique at 150 oC and 200 oC, with
543
which oxygen storage capacity (OSC) and oxygen storage complete capacity (OSCC)
544
are determined and evaluated by the production of CO2 (shown in Fig. 4B) and the
545
consumption of CO (shown in Fig. 4C). For Fig. 4B, at 150 oC, the CuCe sample
546
exhibits the low OSC vlaue of 46.4 μmol CO2/g at the first pulse with arriving the 26 ACS Paragon Plus Environment
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547
maximum value at the fifth pulse, indicating the low oxygen mobility. While after the
548
addition of Fe the corresponding value is 94.8 μmol CO2/g at the first pulse with
549
arriving the maximum value forward to the third pulse, which provides the direct
550
evidence to indicate that Fe doping into CeO2 lattice largely facilitates the migration
551
rate between the highly dispersed oxygen, the surface lattice oxygen and the
552
subsurface lattice oxygen via oxygen vacancies.87,88 With the temperature elevating to
553
200 oC, the oxygen consumption rate obviously enhances, and the oxygen storage
554
capacity is observed to be improved by the incorporation of Fe. These results
555
demonstrate that Fe doping into CuCe sample largely enhances the ability of oxygen
556
migration without the presence of O2, as shown in scheme 1(b). The change in the CO
557
consumption shown in Fig. 4C is consistent with the production of CO2, which also
558
supports the conclusion. Moreover, an interesting phenomenon is observed for the
559
generation of CO2 with the pulsing time proceeding. It is generally speaking that the
560
production of CO2 during the pulsing process goes through one stage, i.e., the amount
561
of CO2 would generate largely at the first pulsing and keeps at a maximum value, and
562
then decrease gradually to zero. However, in our Fig. 4B, the production of CO2 goes
563
through two stages, i.e., the amount of generated CO2 keeps firstly at a low value, and
564
then arrives to a maximum value, afterwards the value reduces to zero. These volcanic
565
and inverted-volcanic curves observed in the production of CO2 and the consumption
566
of CO during the pulsing process indicates the existence of two different types of
567
reactive oxygen species over CuCe and CuCe-Fe samples. One is very highly
568
reactivity to CO and another has relative reactive activity. Combining with the TPR 27 ACS Paragon Plus Environment
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569
results, it is initially inferred that the first active oxygen is attributed to the oxygen
570
bound to the highly dispersed CuOx, and the second active oxygen is assigned to the
571
surface lattice oxygen bound to Cu-[Ox]-Ce species.
572
To prove the above assumption and further investigate the role of the different
573
types of reactive oxygen species on the activity of CO-PROX, a specific pretreatment
574
by 10 % (v/v) H2/Ar at 150 oC (for 2 min over CuCe, P150-2, and for 1 min over
575
CuCe-Fe, P150-1, respectively) is used to eliminate the oxygen bound to the highly
576
dispersed CuOx, and the corresponding TPR, OSC, and the CO oxidation activity over
577
CuCe and CuCe-Fe samples are in-situ characterized without contacting any air. And
578
the corresponding results are shown in Fig. 5A and 5B, Fig. 5C and 5D, Fig. 5E and
579
5F, respectively. In the TPR profiles (Fig. 5A and 5B), it is found that when the CuCe
580
and CuCe-Fe samples are exposed to the 10 %(v/v) H2/Ar at 150 oC for 2 min and 1
581
min, respectively, all the α peaks corresponding to the oxygen bound to the highly
582
dispersed CuOx are completely consumed, and the β peaks characterized by the
583
surface lattice oxygen bound to Cu-[Ox]-Ce species are almost unaffected. These
584
pretreated samples are then in-situ characterized by the CO pulsing processing. It is
585
seen from Fig. 5C and 5D that the volcanic and inverted-volcanic curves of CuCe and
586
CuCe-Fe samples disappear, and the amount of CO2 generates at the first pulsing with
587
a maximum value, and then decrease gradually to zero. This result provides a proof to
588
prove that the oxygen consumed at the first three CO pulsing in the Fig. 4B and 4C
589
should be related to the highly dispersed CuOx, and the oxygen consumed after the
590
three CO pulsing should be associated with Cu-[Ox]-Ce species. It seems to deduce 28 ACS Paragon Plus Environment
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ACS Catalysis
591
that the very high reactive oxygen is connected to the highly dispersed CuOx, which is
592
thought to be preferentially consumed in the CO-PROX process especially at the low
593
temperature. The catalytic activities for CO-PROX over CuCe and CuCe-Fe samples
594
evaluated in-situ after the H2 pretreatment are shown in Fig. 5E and 5F, respectively.
595
The CO oxidation activity is expected to reduce at the low temperature region over
596
CuCe catalyst, which proves that the oxygen bound to highly dispersed CuOx is
597
beneficial to improve the low-temperature CO oxidation activity. Although the CO
598
oxidation activity decreases to some extent, the dominant activity for CO-PROX is
599
not loss, which is an indicative of the importance of the surface lattice oxygen bound
600
to the Cu-[Ox]-Ce species that participate in the CO oxidation process, especially at
601
the relative high temperature region. However, when Fe species is incorporated into
602
CuCe sample, it is strange that there is no decrease in the CO oxidation activity. This
603
result seems to arrive a opposite result that the oxygen bound to highly dispersed
604
CuOx has no influence on the CO oxidation activity. To illuminate clearly this
605
phenomenon, the structural properties change induced by the incorporation of Fe
606
should be considered. As evidenced by XRD, XPS, Raman and H2-TPR that the
607
incorporation of Fe into CeO2 lattice leads to the formation of more oxygen vacancies,
608
which promotes the lattice oxygen migration rate. Thus, we assume that the identical
609
CO oxidation activity before and after the H2 pretreatment is not only due to the
610
migration of the subsurface lattice oxygen to surface lattice oxygen at the relative high
611
temperature (≥100 oC) and but also owing to the migration of gas-phase oxygen to the
612
highly dispersed oxygen during the reaction process at the relative low temperature 29 ACS Paragon Plus Environment
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613
( 300 oC).102,103 CuCe catalysts was reported to have good
788
activity for WGS at the temperature above 200 oC, while only about 5% CO 37 ACS Paragon Plus Environment
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789
conversion was obtained at the temperature blew 200 oC.104-106 If the addition of Fe
790
species into CuCe catalysts could facilitate the occurrence of WGS reaction at blew
791
200 oC, further improving the tolerance of H2O and CO2 as well as the stability of
792
catalysts, is not clear.
793
In general, WGS reaction over CuCe catalyst includes the reaction of CO with
794
surface original -OH groups of catalyst surface as well as the H2O-assisted CO
795
oxidation by the formation of surface -OH groups. Firstly, the reaction of CO with
796
surface original -OH groups of catalyst surface via the WGS type reaction (CO + -OH
797
→ CO2 + 1/2H2) is considered. Since hydrogen (H2) is a probe molecule for the
798
occurrence of WGS-type reaction, thus, CO-TPR was used to detect the presence of
799
H2, and in-situ DRIFTS is conducted to detect the formation of intermediate species.
800
The CO-TPR profiles and in-situ DRIFTS spectra on CuCe and FeCuCe samples are
801
displayed in Fig. 9(A), 9(B), 9(C). As for Fig. 9(A), it is found that the initial
802
temperature for producing H2 is 170 oC for CuCe sample, while the corresponding
803
temperature decreased to 140 oC over FeCuCe sample, suggesting the possible
804
occurrence of WGS-type reaction between surface original -OH and CO at 140 and
805
160 oC due to the promotion role of Fe additive. To investigate the effect of adsorbed
806
water on the WGS-type reaction, fresh FeCuCe was pretreated at 700 oC for 1 h in the
807
atmosphere of pure N2. After the removal of adsorbed water and partial OH groups,
808
the observed temperature increases to 190 oC over FeCuCe sample, indicating that the
809
WGS-type reaction is either facilitated by the adsorbed water or inhibited by the
810
decreased amount of -OH groups. As known, formate species is generally regarded as 38 ACS Paragon Plus Environment
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ACS Catalysis
811
an intermediate species in WGS-type reaction, which would decompose into CO2 and
812
H2 with the temperature raising.81,104,107 In Fig. 9(B) and 9(C), the absorbed formate
813
species characterized by the vibrational frequencies at 2842, 2852, 2932, and 2965
814
cm-1 are distinctly observed for CuCe and FeCuCe samples.81,107,108 It is also seen that
815
larger amount of this intermediate species is formed on FeCuCe at the lower
816
temperature, when compared with the sample of CuCe. Meanwhile, the starting
817
temperature for the decrease in peaks intensity of intermediate species over CuCe is
818
150 oC, while the corresponding temperature for FeCuCe is 170 oC. These results
819
demonstrate the promotional effect of Fe additive on the formation of absorbed
820
formates and on the decomposition of intermediate into CO2 and H2, i.e., WGS-type
821
reaction, which is in well accordance with the CO-TPR results.
822
Secondly, 10% H2O is introduced into the reaction system to consider the
823
H2O-assisted CO oxidation by the formation of surface -OH groups. The independent
824
WGS reactions were performed over CuCe and FeCuCe samples by using 1% CO and
825
10% H2O as reaction gas. CO conversion and products concentration of WGS reaction
826
over two samples are shown in Fig. 10(A) and 10(B), respectively. It is seen from Fig.
827
10(A) that CO conversions of CuCe and FeCuCe samples are close to 0% at 120 oC,
828
and, at higher temperature, the conversion is higher for FeCuCe (10.5% at 140 oC and
829
20.7% at 160 oC, respectively) than that for CuCe (1.2% at 140 oC and 4.9% at 160 oC,
830
respectively), suggesting that WGS reaction occurs at a temperature above 120 oC,
831
and the addition of Fe largely facilitates the WGS reaction. The concentration of
832
products (CO2 and H2) during the course of WGS reaction is further analyzed to 39 ACS Paragon Plus Environment
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833
confirm the promotional effect of Fe additive as well as the reaction mechanism. As
834
shown in Fig. 10(B), it is found that, CO with the concentration of 1046 ppm is
835
consumed at 140 oC (in fact, the difference in the tolerance of H2O and CO2 of the
836
catalysts is significant at this temperature), 488 ppm of CO2 accompanied with 13
837
ppm of H2 is generated over FeCuCe. This result demonstrates that the reaction
838
between CO and surface formed -OH groups is not the main factor promoting the
839
tolerance of H2O and CO2 of the catalysts at 140 oC, since the amount of produced H2
840
is small at this time. Meanwhile, nearly half amount of the generated CO2 compared to
841
the consumed CO confirms that the high tolerance of CO2 and H2O of FeCuCe
842
catalyst at 140 oC (Fig. 8(A) and Fig. 8(B)) is resulted from the possible occurrence of
843
the disproportionation reaction (2CO → CO2 + C). With the temperature increasing to
844
160 oC (the stability tests are performed), when 2071 ppm of CO is consumed, 1268
845
ppm of CO2 accompanied with 379 ppm of H2 is formed over FeCuCe, suggesting
846
that 758 ppm of CO is used for the reaction between CO and surface formed -OH
847
groups, and 1020 ppm of CO is used for the disproportionation reaction. These results
848
evidence that, although the reaction between CO and surface formed -OH groups
849
becomes important, the disproportionation reaction has the slight greater impact on
850
the stability than the latter (1.34 vs 1.0). Thus, the longer-term stability of FeCuCe
851
than CuCe sample at about 160 oC is attributed to the synergistic roles of the
852
disproportionation reaction with the reaction between CO and surface formed -OH
853
groups. With the temperature increasing from 160 oC to the higher one, the reaction
854
between CO and surface formed -OH groups is gradually predominant. 40 ACS Paragon Plus Environment
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ACS Catalysis
855
To further prove the occurrence of the disproportionation of CO as well as the
856
subsequent reaction (C + H2O = CO + H2), which is deemed as another pathway for
857
WGS reaction, the determination of the presence of carbon-containing species over
858
the catalyst surface is an important evidence. Therefore, the CO pulsing technique
859
was introduced to proceed the disproportionation of CO where enough CO pulse was
860
consumed at 140 oC. The pulsed FeCuCe sample was further reacted with 10% H2O at
861
140 oC for 3 h to verify the subsequent reaction (C + H2O = CO + H2). The pulsed
862
CuCe and FeCuCe samples as well as reacted FeCuCe after CO pulsing were
863
characterized by Raman technique so as to determine the presence and the variation of
864
carbon species. The Raman spectroscopy of them are shown in Fig. 10(C), and H2
865
concentration produced during the reaction of pulsed FeCuCe with H2O is also
866
displayed in Fig. 10(D). It is found that no Raman bands corresponding to carbon
867
species are found for pulsed CuCe sample, whereas pulsed FeCuCe sample exhibits
868
distinct Raman peaks at 1360-1400 cm-1, and 1480-1520 cm-1 as well as
869
1580-1640cm-1, attributed typically to the different type of unorganized carbonaceous
870
species,72,109-111 which evidences the occurrence of the disproportionation of CO.
871
After the reaction between the pulsed FeCuCe sample with deposited carbonaceous
872
species and H2O, the amount of deposited carbonaceous species obviously decreases
873
(Fig. 10C-(c)), and meanwhile, a small number of H2 is generated (Fig. 10(D)),
874
demonstrating unambiguously the consumption of carbonaceous species by the
875
presence of H2O (C + H2O = CO + H2). These carbonaceous species are generally
876
under development, and thus can be reacted with H2O to generate CO and H2 under 41 ACS Paragon Plus Environment
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877
the current reaction condition. However, large amount of H2 cannot be produced
878
possibly because that the reaction of carbonaceous species with H2O is a
879
rate-determining step, which is favorable in the long-term stability test instead. This
880
slow reaction rate of deposited carbonaceous species with H2O and the fast reaction
881
rate of the disproportionation reaction could lead to the generation of small amount of
882
H2 as well as large amount of CO2, which can also explain the situation that the
883
selectivity of H2 decreases at 140 oC in Fig. 8(B) (CO2 selectively increases) as well
884
as the selectivity of H2 increases at about 160 oC in Fig. 8(C) (CO2 selectively
885
decreases).
886
In summary, considering that the conversions of CO in WGS reaction are
887
approximately 10 % at 140 oC and 20% at 160 oC over FeCuCe, the primary causes of
888
the high tolerance of H2O and CO2 towards FeCuCe catalyst at 140 oC and the
889
long-term stability of FeCuCe around 160 oC is due to the promotional effect of Fe
890
additive on the lattice oxygen mobility, the second one is the important role of the
891
WGS reaction, which is driven by the addition of Fe. Two reaction pathways
892
(mechanism) are considered to be occurred during the course of WGS reaction. The
893
disproportionation of CO is responsible for the high tolerance of H2O and CO2 of
894
FeCuCe catalyst at 140 oC, and WGS reaction by the disproportionation pathway as
895
well as the H2O-assisted CO oxidation by the formation of surface -OH groups are
896
beneficial to the long-term stability of FeCuCe. However, the decomposition of
897
surface carbonate by H2O is not regarded as the main reason contributing to the high
898
tolerance of H2O and CO2 and the excellent stability of the catalyst, as confirmed 42 ACS Paragon Plus Environment
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ACS Catalysis
899
(Section 5 of SI).
900
4. CONCLUSIONS
901
In summary, CuCe and Fe doped CuCe catalysts were prepared via a solvent-free
902
combustion synthesis (SFCS) method to investigate the active species and the Fe
903
doping effect. The active Cu1+ species was demonstrated by in-situ DRIFTS technique
904
and XPS as well as CO-TPD to be adsorption sites for the adsorption and the
905
activation of CO molecules. The modification of Fe additive resulted in the
906
incorporation of CeO2 lattice so as to induce the redox chemistry between Fe3+/Fe2+
907
and Cu1+/Cu2+, which appropriately enhanced the adsorption ability of CO over Cu1+
908
adsorption sites. Our combined in-situ OSC and designed TPR characterizations
909
showed that the surface highly dispersed oxygen species related to CuOx species was
910
confirmed as one of active species and to be involved in the CO oxidation at low
911
temperature. The surface lattice oxygen species assisted with Cu-[Ox]-Ce (Fe-O-Ce)
912
was also proved to participate into the reaction process. Due to the charge
913
compensation and the difference in the ionic radius between Fe and Ce ions, the
914
incorporation of Fe into CeO2 lattice generated plentiful oxygen vacancies, which was
915
not only in favor of trapping gas-phase oxygen to form the surface highly dispersed
916
oxygen, but also promoted the migration of subsurface lattice oxygen to surface active
917
oxygen. The former promoted the CO catalytic activity (TOF value increased from
918
3.62×10-2 s-1 for CuCe to 4.50×10-2 s-1 for CuCe-Fe), and the latter in combination
919
with the high resistance to H2O using Fe doped as favourable WGS catalyst made an
920
important contribution to the long stability in the presence of CO2 and H2O (increased 43 ACS Paragon Plus Environment
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921
the lifetime from 170 to 400 h).
922
● Supporting Information
923
The Supporting Information is available free of charge on the ACS Publications
924
website.
925
Additional Transmission electron microscope (TEM). H2-TPR profile of CuO.
926
In-situ DRIFTS of CuCe and FeCuCe samples in the gas stream with the addition of a
927
certain amount of H2O. The method of calculating the concentration of oxygen
928
vacancies, and the explanation of satellite peak of Fe 2p XPS spectra.
929
930
Corresponding Authors
931
* E-mail:
[email protected] 932
Notes
933
The authors declare no competing financial interest.
934
935
We gratefully acknowledge the financial supports of National Natural Science
936
Foundation of China (Grant No. 21267011, 21367015, 21666013, 51704137 and
937
U1402233).
AUTHOR INFORMATION
ACKNOWLEDGMENTS
938
44 ACS Paragon Plus Environment
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939
ACS Catalysis
Table 1 Textural parameters and structural properties of CuCe and CuCe-Fe catalysts.
Samples
TOF×
r(CO)×10-6
lattice parameter
crystallite
10 (s )
(mol g s )
CuCe-Fe
4.50
1.60
0.54016
9.6
CuCe
3.62
1.13
0.54126
11.0
-2
-1 a
-1 -1
size (nm)
Cu/Ce
Vp
d
(m g )
(cm g )
0.336
19.7
0.042
0.293
12.6
0.044
c
(nm) b
SBET
Concentration of
2
-1
3
Isat/Imp
d
oxygen vacancies
Ce3+/(Ce3++C e
e ) (3d5/2) 4+
-1
(%)
0.32
4.43
27.56
51.65
0.19
3.84
22.00
36.33
a
TOF value was calculated based on the CO conversion at 80 oC,
941
b
Lattice parameters were obtained by using MDI Jade 5.0 software according the data of XRD,
942
c
Crystallite size determined from XRD by the Scherrer’s equation with the half width of the diffraction peak of the (111) plane,
943
d
Values were estimated from XPS peak fitting data,
944
e
The concentration of oxygen vacancies (N) was calculated by using Raman spectroscopy.
ACS Paragon Plus Environment
OII) d (%)
(×1021 cm-3)
940
45
OII/(OI +
d
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
945
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Figure captions Fig. 1 (a) CO conversion and (b) CO2 selectivity of CuCe and CuCe-Fe. Reaction conditions: mcat=300 mg, feed gas: 1% CO, 1% O2, 50% H2 and balance He, flow rate=80 mL/min. Fig. 2 (A) XRD patterns of CuCe and CuCe-Fe. X-ray photoelectron spectra of CuCe and CuCe-Fe in the Ce 3d region (B), Fe 2p region (C) and Cu 2p region (D). Fig. 3 (A) Raman spectroscopy and (B) O 1s XPS spectra of CuCe and CuCe-Fe. Scheme 1. Structural model for CuCe (a) and CuCe-Fe (b) samples as well as the corresponding reaction mechanism for special CO oxidation as main reaction in CO-PROX. Fig. 4 (A) H2-TPR profiles of CuCe and CuCe-Fe. Comparison of oxygen storage capacity (OSC) in the form of CO2 (B) and CO (C) under mild conditions of T=150 and 200 oC for CuCe and CuCe-Fe. Fig. 5 Designed TPR characterization of CuCe (A) and CuCe-Fe (B) samples, P150-0: the fresh samples, P150-1 and P150-2: after pretreatment at 150 oC for 1 min, P150-1-R160 and P150-2-R160: the pretreated samples were reacted from 60 to 160 C every 20 oC, and P150-1-R220 and P150-2-R220: the pretreated samples were
o
reacted from 60 to 220 oC every 20 oC. OSC measurements of the pretreated CuCe and CuCe-Fe samples calculated in the form of CO2 (C) and CO (D). Comparison of the catalytic activity of CuCe (E) and CuCe-Fe (F) before (black line) and after (red line) pretreatment without contacting any oxygen gas. Reaction conditions: mcat=100 mg, feed gas: 1% CO, 1% O2, 50% H2 and balance He, flow rate=80 mL/min. 61 ACS Paragon Plus Environment
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Fig. 6 Designed in-situ TPR characterization of CuCe (A) and CuCe-Fe (B) samples before (gray line) and after (red line) one CO pulse was introduced without contacting any oxygen gas. Fig. 7 (A) CO-TPD of CuCe and CuCe-Fe. In-situ DRIFTS spectra of CuCe (B) and CuCe-Fe (C) recorded under the reaction stream of 1% CO/1% O2/50% H2/He at the elevated temperature. Fig. 8 (A) CO conversion and (B) CO2 selectivity of CuCe and CuCe-Fe with the coexistence of H2O and CO2. (C) The stability test with time on stream for CuCe at 160 oC and CuCe-Fe at 155 oC, respectively. mcat=300 mg, feed gas composition: 1% CO, 1% O2, 50% H2, 15% CO2, 10% H2O and He as balance gas, flow rate=80 mL/min. Fig. 9 (A) CO-TPR profiles of fresh CuCe and FeCuCe samples as well as the pretreated FeCuCe sample (calcined in-situ in the atmosphere of pure N2 at 700 oC for 1 h). In-situ DRIFTS spectra of CuCe (B) and CuCe-Fe (C) recorded at the range of 2500-3200 cm-1 under the reaction stream of 1% CO/1% O2/50% H2/He. Fig. 10 (A) CO conversion and (B) products concentration of WGS reaction over CuCe and CuCe-Fe samples. Reaction conditions: mcat=300 mg, feed gas: 1% CO, 10 % H2O, and balance N2, flow rate=80 mL/min. (C) Raman spectroscopy of (a) pulsed CuCe (all the oxygen was consumed by CO pulsing), (b) pulsed FeCuCe, (c) pulsed FeCuCe after the reaction with 10% H2O at 140 oC for 3 h (balance with pure N2 to keep the flow rate of 80 mL/min). (D) H2 concentration of pulsed FeCuCe sample after the reaction with 10% H2O at 140 oC for 3 h. 62 ACS Paragon Plus Environment
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Page 63 of 74
CO Conversion (%)
100
(A)
80
CuCe CuCe-Fe
60 40 20 80
120
160 o
Temperature ( C)
100
CO2 Selectivity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
200
(B)
80 60
CuCe CuCe-Fe
40 20 80
120
160 o
200
Temperature ( C)
Fig. 1 Lu et al
63 ACS Paragon Plus Environment
ACS Catalysis
(A)
CuO
CuCe-Fe
CeO2
CuCe
27
29
30
30
40
50
60
70
2-Theta (degree)
80
890
900
Fe 2p1/2
Fe
3+
713.4
Fe
2+
710.0
3+
Fe (sat) 719.4
910
Binding Energy (eV)
(D) Fe 2p3/2
u''
u'
v' v''
880
u'''
v''' u
CuCe v
(C)
Fe 2p
740
31
CuCe
20
28
CuCe-Fe
Intensity (a.u.)
Intensity (a.u.)
(B)
CuCe-Fe
Intensity (a.u.)
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 64 of 74
Cu LMM
+
Cu (569.7) Fe-CuCe
Cu 2p
CuCe
1+
Cu 2+ 932.2 Cu 933.4
2+
Fe (sat)
920
580
952.6
570
952.6
560
FeCuCe CuCe
shake-up
716.3
730
720
Binding Energy (eV)
710
930
940
950
Binding Energy (eV)
960
Fig.2 Lu et al
64 ACS Paragon Plus Environment
0.75
(A)
A599/A459
CuCeFe
300
0.60
463 459
0.45 0.30
CuCe CuCe-Fe
0.15 0.00
CuCe CuCe-Fe
599
CuCe
400
500
600
-1
700
800
Raman shift (cm ) 529.3 (B)
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Normalized intensity (a.u.)
Page 65 of 74
CuCe-Fe
+0.4
528.9 OII
OI 526
O 1s
528
530
532
Binding Energy (eV)
CuCe 534
536
Fig. 3 Lu et al
65 ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Scheme 1 Lu et al 66 ACS Paragon Plus Environment
Page 67 of 74
(a)
H2 Consumption (a.u.)
H2 Consumption (a.u.)
CuCe CuCe-Fe
(A)
β
α
100
200
300
400 500 600 700 o Temperature ( C)
800
300 400 o Temperature ( C)
500
700
(B)
OSC (umol CO2/g)
600 500
CuCe-150 FeCuCe-150 CuCe-200 FeCuCe-200
400 300 200 100 0
0
2
4
6
8
10
Pulse number
12
14
16
18
1000
OSC (umol CO/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
800
CuCe-150 FeCuCe-150 CuCe-200 FeCuCe-200
600 400 200 0
(C) 0
2
4
6
8
10
Pulse number
12
14
16
18
Fig. 4 Lu et al
67 ACS Paragon Plus Environment
200 300 o Temperature ( C)
600 500 400 300 200 100 0
CuCe-P150-2min FeCuCe-P150-1min
0 2 4 6 8 10 12 14 16 18 Pulse number
1000 P150-1-R220 P150-1-R160 P150-1 P150-0
200 300 o Temperature ( C)
(a) CuCe
40
(b) CuCe-150-2min
20 0
60
80 100 120 140 160 o Temperature ( C)
(C)
600 400
CuCe-P150-2min FeCuCe-P150-1min
200 0
400
80 60
800
(E)
100
(D) Conversion of CO (%)
700
400 100
(B)
OSC (umol CO/g)
P150-2-R220 P150-2-R160 P150-2 P150-0
CuCe-Fe
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0 2 4 6 8 10 12 14 16 18 Pulse number (F)
100
Conversion of CO (%)
(A) H2 Consumption (a.u.)
CuCe
100
OSC (umol CO2/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
H2 Consumption (a.u.)
ACS Catalysis
80
(a) FeCuCe
60 40
(b) FeCuCe-150-1min
20 0
60
80 100 120 140 160 o Temperature ( C)
Fig.5 Lu et al
68 ACS Paragon Plus Environment
Page 69 of 74
H2 Consumption (a.u.)
(A)
CuCe CuCe-one pulse
100
200
300 o
400
Temperature ( C)
(B)
H2 Consumption (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
CuCeFe CuCeFe-one pulse 100
200
300 o
400
Temperature ( C)
Fig.6 Lu et al 69 ACS Paragon Plus Environment
ACS Catalysis
Intensity (a.u.)
(A) (b) CuCe-Fe
(a) CuCe 50
100
150
200
Temperature ( C)
(B)
1579
Absorbance (a.u.)
1294
o
o
210 C o 190 C o 170 C o 150 C o 130 C o 110 C
250
300
CuCe
2105
o
90 C o
70 C o
50 C o
30 C
1200
1600
2000
2400
-1
Wavenumber (cm )
(C) 1294
Absorbance (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1579
o
210 C o 190 C o 170 C o 150 C o 130 C
2100
CuCe-Fe
o
110 C o
90 C o
70 C o
50 C o
30 C
1200
1600
2000 -1
2400
Wavenumber (cm )
Fig.7 Lu et al
70 ACS Paragon Plus Environment
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(A) 100
CO Conversion (%)
80 60
CuCe with H2O and CO2
CuCe-Fe with H2O and CO2
40 20 100
(B)
120
140
160
o
Temperature ( C)
80
CO2 Selectivity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
60 40
CuCe with H2O and CO2 CuCe-Fe with H2O and CO2
20 100
(C)
120
140
160
o
Temperature ( C)
100
CO Conversion
80 CO2 Selectivity
60
CuCe-Fe CuCe
40 20
0
100
200
Time (h)
300
400
Fig. 8 Lu et al
71 ACS Paragon Plus Environment
ACS Catalysis
(A) H2 signal (a.u.)
CuCe FeCuCe FeCuCe o (700 C 1h)
80
120
160
200
240
o
280
Temperature ( C)
(B)
2852 2965 2842 2932
Intensity (a.u.)
CuCe
2600
(C)
FeCuCe
2800
3000 -1
Wavenumber (cm )
2856 2967 2843 2935
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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210 190 170 150 130 110 90 70 50 30 3200
210 190 170 150 130 110 90 70 50 30
2600
2800
3000 -1
3200
Wavenumber (cm )
Fig. 9 Lu et al
72 ACS Paragon Plus Environment
CO Conversion (%)
100
(A)
CuCe FeCuCe
80 60 40 20 0 120
140
160
180
200
o
220
240
5000
CuCe-CO2
(C)
4000
CuCe-H2
3000
FeCuCe-H2
2000 1000
260
0 120
140
160
H2 concentration (ppm)
(c)
1360-1400
o
(a) Pulsed CuCe at 140 C o
(b) Pulsed FeCuCe at 140 C (c) Reacted FeCuCe
1000
1200
1400
1600 1800 -1 Raman Shift (cm )
200
o
10
(b)
1480-1520
180
220
240
260
Temperature ( C)
(a) 1580-1640
(B)
FeCuCe-CO2
Temperature ( C)
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Products Concentration (ppm)
Page 73 of 74
2000
(D)
8
FeCuCe sample after the CO pulsing
6 4 2 0
0
40
80
120
Time (min)
160
200
Fig. 10 Lu et al
73 ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
297x112mm (300 x 300 DPI)
ACS Paragon Plus Environment
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